Organosilicon compounds have been employed for some time in the treatment of inorganic oxide surfaces, such as inorganic oxide films, particulate fillers and pigments, and fibers (such as glass fibers, steel fibers and aluminum fibers). Aluminum and steel fibers are regarded to be oxide surfaces because they are oxidized even though their subsurfaces are not. The typical organosilicon treatment involves coating such a surface with a hydrolyzate (and/or condensate of the hydrolyzate) of an organofunctional hydrolyzable silane. As a rule, the treatment enhances bonding between the inorganic oxide surface and resinous media and, consequently, has utility as a primer treatment in the application of coatings, adhesives or sealants to inorganic oxide surfaces and as a filler pretreatment to improve the strength and structural integrity of filled resin composites such as glass fiber reinforced plastics. Such organofunctional hydrolyzable silanes are termed "Coupling Agents" or "Adhesion Promoters."
In addition to the improvement of bonding, coupling agents have found other uses related to their ability to alter the surface characteristics of inorganic oxides, such as their application to glass fibers as a size during processing and their use as dispersing aids for inorganic oxide pigments (e.g., titanium dioxide).
Another conventional technique for supplying the coupling agent to the inorganic oxide surface is by the integral blending technique. This technique involves adding to the resin medium the desired amount of the coupling agent and providing the medium in contact with the inorganic oxide surface by supplying the latter as a particulate filler or fiber to the medium or supplying the medium with the coupling agent to a continuous surface of the inorganic oxide in the form of a film, fabric, foil or other shapes, wherein the coupling agent migrates within the medium to contact the inorganic oxide surface or surfaces, react thereat and couple with the medium under the molding, curing and other shaping conditions.
The organofunctional hydrolyzable silane contains at least one, and as many as three, hydrolyzable groups, each of which is bonded to a silicon atom. Typical hydrolyzable groups include alkoxy of 1 to about 4 carbon atoms, alkoxyalkoxy containing up to about 6 carbon atoms, halogen such as chlorine, fluorine and bromine, acyloxy of 2 to about 4 carbon atoms, phenoxy and oxime. The preferred hydrolyzable groups are alkoxy, alkoxyalkoxy and acryloxy. Common organofunctional groups are bonded to the silicon by a carbon to silicon bond. The typical commercial functional radicals present in the organofunctional groups are vinyl, methacryloxy, primary amino, beta-aminoethylamino, glycidyl, epoxycyclohexyl, mercapto, polysulfide, ureido, and polyazamide. The organofunctional groups are usually quite small in relation to the molecules of the resin medium.
The coupling agent is typically supplied to the surface of the inorganic oxide material as a hydrolyzate in the presence of a hydrolyzing agent such as dilute acetic acid or sodium hydroxide solution. The coupling agent molecules are subsequently bonded to the inorganic oxide surface through siloxy moieties (.tbd.Si--O--) by condensation of the silanol groups of the coupling agent and the hydroxyl groups of the inorganic oxide material. Additionally, the silicon atoms of individual coupling agent molecules may become bonded to each other through oxy moieties (--O--) by a similar process of hydrolysis and condensation. It is not necessary to supply a hydrolyzing agent to the inorganic oxide surface if sufficient ambient moisture is present at the inorganic oxide surface to effect hydrolysis of the hydrolyzable groups. The coupling agent becomes bonded to the resinous medium through a chemical reaction between the reactive organofunctional group of the coupling agent and the complementarily reactive groups in the medium.
Those skilled in the art are fully aware that there are a number of problems associated with the use of the coupling agents of the prior art, particularly with their use in conjunction with non-reactive resin media. The bonds between the coupling agent and the inorganic oxide surface can be disrupted by hydrolysis in the presence of water. Likewise, the bonds between silicon atoms of individual coupling agent molecules are subject to attack by hydrolysis. This problem is compounded when the coupling agent is used in a non-reactive resin in that only minimal bonding, primarily of an associative or van der Waals type, can be achieved between the medium and the coupling agent. Thus, the wet strength of resin/inorganic oxide composites using coupling agents of the prior art is particularly poor when the coupling agent is used with a non-reactive resin medium. This may be due, at least in part, to the displacement of coupling agent molecules from the inorganic oxide surface under hydrolysis conditions, wherein the bonds between the coupling agent and inorganic oxide surface and the bonds between the silicon atoms of individual coupling agent molecules are disrupted by hydrolysis and the weak associative bond between the resin medium and the coupling agent is insufficient to prevent displacement of the coupling agent. Moreover, since bonding between the resin medium and the coupling agent occurs substantially at the interface, a boundary layer is created along which cleavage can occur.